Overview of Open Systems Interconnection Model (called OSI Model)

Open Systems Interconnection (or OSI) is a conceptual model created by International Organization for Standardization (ISO) to describe a stack of layers that enable communications between two systems. It establishes a framework consisting of seven layers that would make the communication feasible between the two systems.


Purpose of OSI Model

Outline a set of design standards for a framework that would make data transmission between two devices feasible. The standard specifications would allow Hardware manufacturers and Software developers to build components that are able to smoothly exchange data with each other.


OSI Model


Layers of OSI Model


Layer 1 – Physical

This layer is responsible for sending and receiving raw data between a device (such as switches) and a physical medium (such as coaxial cable). It handles conversion between raw data and signals (like electrical impulse).


Layer 2 – Data Link

This layer is responsible for data transmission between two devices on the same Network. The layer manages the data coming from Physical layer, including detecting of potential errors and attempts to correct the errors.

This layer is complex, and has two further sub-layers:

  • Medium Access Control (MAC) layer
  • Logical Link Control (LLC) layer


Layer 3 – Network

The previous two layers – Layer 1 and 2 operate within the same Network. It is Layer 3 (Network layer) that connects two different Networks together. It is responsible for data transmission between two networks, and does so in forms of packets, by breaking up bigger data into packets. Network layer reassembles this set of packets on the receiving side of the Network. Internet would not have been possible without this Network protocols in place.


Layer 4 – Transport

Layer 4 is responsible for data transmission and data coordination between two devices. It breaks the data received from Session layer into smaller chunks for consumption by Network layer, and then reassembles on the receiving device. This layer also has a mechanism to control flow (targets, speed, etc.) and reliability (error control) of data exchanged. TCP (of TCP/IP stack) corresponds to this layer.


Layer 5 – Session

Session layer is responsible for handshake between two systems, and thus opening, maintaining and closing the connection between the two systems. This layer also deploys a mechanism to perform checkpoints on data exchange, and in case of errors, it is able to perform recovery / resume from checkpoints rather than starting data send all over from the beginning.


Layer 6 – Presentation

This layer is a sense is Syntax layer – that is – this layer performs the interpretation of data between what applications (at Application Layer) are able to understand and data format that Session layers use to start the data transmission. This layer has a mechanism to format, compress and encrypt the data being transferred.


Layer 7 – Application

Application layer is the closest to the user, visible to the user (via a User Interface). This layer exists to allow users to interact with a software and perform various types of data exchanges. Examples – Telnet, or Browser, or Email Client.